Atomic Layer Deposition (ALD) (also referred to as Atomic Layer Epitaxy, or ALE) is a Chemical Vapor Deposition (CVD) technique. Classical CVD is based on exposing a surface to a gas mixture. The surface is heated, and the heat induces a reaction and/or decomposition of the gases. This results in the formation of a solid film layer on the surface.
ALE is a species of ALD and refers to the growth of a crystalline film on the substrate surface, while ALD is more general and extends to the deposition of amorphous materials as well. In this specification the term ALD will be generally used, with the understanding that it includes ALE as well.
The principle of ALD is that the raw materials are two gases, which will be referred to as “a” and “b”. The gases “a” and “b” are so mutually reactive that they cannot be present at the same time in a gas mixture, as they would spontaneously react, leading to self-ignition or formation of dust. To form the film, the substrate is alternately exposed to the two gases. Care is required to ensure that the first gas is adequately eliminated from the substrate environment before the second gas is introduced to the substrate environment. A fraction of the first gas is chemisorbed as a mono-layer on the substrate surface, and it is this chemisorbed fraction of the first gas that reacts with the second gas to form the growing layer on the surface. Given sufficient exposure time, the chemisorbed layer of gas “a” will fully react with gas “b” creating a complete monolayer of solid compound and gaseous by-products. By removing the second gas from the substrate environment, exposing the substrate to the first gas, and repeating the process, the film is grown by an increasing number of atomic layers. Note that during a chemical exposure, the gas reactivity can be boosted by plasma excitation.
In the past, two basic techniques have been used to separate the reactive gases in ALD. These are referred to as the space separation method and the time separation method.
In the space separation method, the substrate is physically moved from an environment or zone where gas “a” is present to another chemically decoupled environment or zone where gas “b” is present.
In the time separation method, the substrate remains in one chamber and is exposed in successive independent steps to gases “a” and “b”. In between additions of the reactive gases, the substrate environment is evacuated by pumping and/or is flushed with a nonreactive gas.
The deposition rate obtained by ALD is low, with a typical rate of 0.1 angstroms/sec. or less. A process time of up to 30 minutes can be required to deposit a layer of 15 nm. As a result, ALD processes tend to have low production rates and tend not to be cost effective.
A major factor in the long cycle time for time separated ALD is the degassing step in which one reactant is removed from a reaction chamber prior to exposure to the second reactant. In this method, the degassing must remove the reactant gas from the reaction chamber environment, including the gas delivery subsystem. In contrast, when space separation is used, the degassing time is reduced, as degassing is limited to the substrate and the substrate holder or susceptor. For maximum efficiency in a space separated ALD process, quick and effective degassing of the substrate and holder must occur during the transfer of the substrate from zone “a” to zone “b”.
Failure to effectively degas the reaction environment or remove the reactant gas “a” from the substrate prior to exposure to reactant gas “b” will result in unwanted gas phase reactions leading to non conformal deposition or to dust formation, as the mingling gases will produce freely floating particles of the material that is supposed to be deposited on the substrate.
Careful degassing in space or time separated ALD usually avoids dust formation in the reactive zone proximate to the substrate. However, in systems utilizing a single pump downstream of the reactive zone or zones, dust may form downstream toward the pump or in any system dead volume that is not well managed in terms of chemical inter-diffusion. Accumulation of dust, even downstream of the reaction zones, creates a risk for contamination of the substrates in the reactive zone or zones. While careful engineering and maintenance can avoid dust contamination of substrates, the interruptions for regular maintenance required to avoid dust contamination will further reduce the output and cost-effectiveness of the ALD process.
In addition, periodic maintenance is required to remove deposition film from the interior of reaction chambers used in time separation ALD methods. Because the walls of the chamber are also exposed to alternating reactive gasses, a layer of ALD film tends to form on the reactor chamber's walls. The rate of deposition can be reduced by lowering the temperature of the reactor chamber's walls, but this tends to also slow the rate of degassing, hence the ALD cycle rate, further reducing deposition rate efficiency. The need for this type of periodic maintenance is reduced in reactors based on space separation ALD. Because the chamber walls are not subjected to alternating exposure to reaction gasses “a” and “b”, the chambers do not build up a layer of the deposition film. The substrate carrier will still be exposed to the alternating reactive gas environments, but cleaning of the substrate carrier entails much less downtime for the ALD apparatus than cleaning of reactor chambers.
One approach to space separation has been the use of a susceptor in the form of a turntable which carries a substrate through a number of different and non-compatible chemical environments. This approach has been used when the gases in the non-compatible chemical environments are moderately reactive. ALD processes for use in semiconductor integrated circuit manufacturing, however, will require the use of gases which are strongly mutually reactive in order to create deposited layers meeting required standards of thickness control and conformality. Currently available turntable designs are unsatisfactory for use with such highly reactive gases.
FIG. 1 shows a prior art turntable apparatus. The apparatus 9 contains two reactive enclosures 24 and 34 contained in an outer enclosure 11. Reactive gas inlets 20 and 30 supply reactive gasses to reaction zones 2 and 3 in reactive enclosures 24 and 34, respectively. Although not indicated in the drawing, at least one of the zones 2 and 3 may also comprise plasma excitation by any of the known means. A turntable 7 is rotatable on a support arm 60. The substrates 8 rest on the turntable and so are alternately transported through the reactive enclosures 24 and 34. A neutral gas inlet 40 supplies gas to a buffer zone 6, which surrounds the reactive enclosures 24 and 34. During the CVD process the rotation of the turntable exposes the substrates 8 successively to the reactive gases in the reaction zones 2 and 3, building up a CVD film. The pressure of the reactive gasses in the reaction zones 2 and 3 is kept higher than the pressure of the neutral gas in the buffer zone 6. During the process, reactive gas in the reactive zone 2 escapes the reaction enclosure 24 through conductance slits or gaps 21 into the buffer zone 6. At the same time, reactive gas in the reactive zone 3 escapes the reaction enclosure 34 through conductance slits or gaps 31 into the buffer zone 6. The neutral gas and reactive gases in the buffer zone 1 are vented through the pipe 5 toward a common pump 4. The aim of this design is to create a dynamic gas flow running outward at the periphery of the reactive zones which prevents the contaminated gas from the buffer zone 1 from backstreaming into the reactive zones. In this design, however, both excess reactive gases “a” and “b” end up in the buffer zone where they will generate some film and some dust. Because the buffer zone is flushed with neutral gas, the build up of film and dust will take time. However, in the long run, such a system will have a contamination problem, in particular when using highly mutually reactive gases for conformal ALD growth. Thus, this design is adequate for protecting the chemical purity in the reactive zones, but is inadequate for preventing dust contamination of the system.
It would be desirable to have an ALD apparatus for use in space separation ALD that would allow the use of highly mutually reactive gasses and that would minimize the formation of dust downstream of the reaction chambers.